Field of Invention
The present invention is generally directed to a system for providing neuro-stimulation, and more particularly, to a system that employs electrical current and/or mechanical vibration to deliver subthreshold and/or aperiodic stimulation to enhance detection and communication of sensory information.
Description of the Related Art
The nervous system of mammals is a complex set of interrelated and interacting sub-systems. The sub-systems are categorized and named both by their anatomic positions and by their function. At the highest level, the nervous system is divided into central and peripheral nervous systems. The central nervous system (CNS) is comprised of the brain and spinal cord; the peripheral nervous system (PNS) subsumes all the remaining neural structures found outside the CNS. The PNS is further divided functionally into the somatic (voluntary) and autonomic (involuntary) nervous systems. The PNS can also be described structurally as being comprised of afferent (sensory) nerves, which carry information toward the CNS, and efferent (motor) nerves, which carry commands away from the CNS.
Interconnections between afferent and efferent nerves are found in the spinal cord and brain. Taken together, certain groupings of afferent and efferent nerves constitute sensorimotor “loops” that are required to achieve coordinated movements in the face of perturbations from the environment and changes in volitional intent. In the periphery (trunk, upper extremities, and lower extremities), afferent nerves carry sensory information arising from special neurons that are sensitive to pain, temperature, and mechanical stimuli such as touch and vibration at the skin surface, and position, force, and stretch of deeper structures such as muscles, tendons, ligaments, and joint capsule. The term “proprioception” generally applies to sensory information directly relevant to limb position sense and muscle contraction. Combined with tactile (touch) sensation, mechanical sensory information is collectively known as “somatosensation.”
Specialized “mechanoreceptor” neurons transduce mechanical stimuli from the body's interaction with the environment into electrical signals that can be transmitted and interpreted by the nervous system. Pacinian corpuscles in the skin fire in response to touch pressure. Muscle spindles, found interspersed in skeletal muscle tissue, report on the state of stretch of the surrounding muscle. Golgi tendon organs sense the level of force in the tendon. Free nerve endings in structures surrounding joints (ligaments, meniscus, etc.) provide additional information about joint position. Some of these mechanoreceptor systems are thought to interact directly via excitatory and inhibitory synapses and descending pathways to modulate the performance or interpretation of signals from other mechanoreceptor systems.
Sensory cells of all types are typically threshold-based units. That is, if the stimulus to a sensory cell is of insufficient magnitude, the cell will not activate and begin signaling. Such a stimulus is called “subthreshold.” A stimulus that is above the threshold is called “suprathreshold.”
Connections within the nervous system-brain, spinal cord, and peripheral nerves are highly changeable in the face of demands placed on the body. New forms of activity, pathologies, and injuries all can lead to durable changes, both beneficial and deleterious, in the nervous system. In healthy individuals, these neurological changes allow for the acquisition of new physical skills, a process termed “motor learning.” Following certain types of soft tissue injury (e.g. rupture of the anterior cruciate ligament of the knee, a structure known to be rich in mechanoreceptors), and subsequent medical efforts such as surgery used to repair the damage, the nervous system can undergo compensatory changes to accommodate for loss of the natural sensory neurons. Similar PNS and CNS nervous system changes account for some individuals' ability to regain lost motor function following spinal or brain injuries. Taken together, these structural changes in the nervous systems—the creation of new useful interconnections or the pruning away of unused pathways—are termed “neuroplasticity” or “neuroplastic changes.”
Recent research has established that afferent (sensory) activity from the periphery is one of the key drivers of neuroplastic changes in the nervous system, both in the PNS and CNS.
Stimulation below perception levels (i.e. subthreshold stimulation) used to enhance the function of sensory cells is described in U.S. Pat. Nos. 5,782,873 and 6,032,074 to Collins, the entire contents of which are incorporated by reference. Collins discloses a method and apparatus for improving the function of sensory cells by effectively lowering their threshold of firing. Briefly, a subthreshold stimulation, or subsensory stimulation or “bias signal,” is input to the sensory neuron thereby predisposing the neuron to firing, without actually causing it to fire. In some embodiments, the stimulation may have an aperiodic waveform. In one particular embodiment, the bias signal is a broadband signal containing many frequencies, often termed “white noise.” Since sensory cells are typically threshold-based units, lowering the sensory cell threshold decreases the level of outside stimulus needed to cause the sensory cell to respond (i.e. fire). Thus, the sensory cell, in the presence of the bias signal, is expected to respond to stimulus intensities that would normally be considered subthreshold to the neuron in the absence of noise. Both electrical and mechanical modalities of bias signal, used individually or in combination, may be used to effect the lowering of sensory neuron detection threshold. In other words, the stimulation essentially energizes sensory neurons based on a principle termed “stochastic resonance” (SR), so that they are predisposed to fire in response to stimuli from the environment. By increasing the sensitivity of mechanoreceptors, it is possible effectively to boost the flow of sensory information traveling from muscles, joints, and skin to the body's control centers in a fashion that is concordant with normal function.
One exemplary clinical use of increased sensory information is in the rehabilitation of individuals who suffer loss of sensorimotor function following stroke. According to the American Stroke Association, stroke is the leading cause of serious, long-term disability in the U.S., with the annual cost of stroke-related care expected to exceed $58 billion in 2006. Approximately 700,000 cases of stroke occur each year in the U.S. As a result, over 460,000 patients a year are left with motor impairments, the most common of which is hemiparesis, a weakness or partial paralysis of the body. In addition, a majority of the 5.5 million stroke survivors in the U.S. have some degree of impairment. While many patients improve with current physical rehabilitation therapy, most are left with significant motor deficits. Full recovery from stroke is uncommon. Thus, additional techniques for reversing the motor deficits caused by stroke are necessary. Boosting sensory traffic using the present invention is one such technique. A similar exemplary clinical use is physical rehabilitation for individuals who have suffered traumatic brain injury. Further exemplary clinical uses arise in treatment of individuals who have a temporary or permanent loss of sensory function resulting from aging, disease, or physical injury. For such individuals, the therapy is directed less toward driving neuroplastic changes and more toward providing an ongoing sensory boost as a palliative treatment for a chronic sensory condition.